The present invention relates to an integrated structure of polyacetal resin molding and thermoplastic resin molding as integrated together and also to automobile parts, electric and electronic equipment parts, OA-related parts, various parts of industrial sundries, etc., composed of the integrated structure.
Polyacetal resin has distinguished mechanical strength, creep characteristics, lubricating characteristics and electrical characteristics and thus is widely used, for example, in automobile parts, electric and electronic equipment parts, OA-related parts, various parts of industrial sundries, etc. Particularly due to such advantages as superior resistance of polyacetal resin to fuel, polyacetal resin is much used in bracket-materials for in-tank type fuel pump systems, fuel tank flanges, or automobile fuel tank-related parts such as valves, etc., which are disclosed in JP-A-8-279373.
Nowadays, total-amount control over hydrocarbon emission from automobiles (so called Vehicle Evaporative Emission Regulation) is under study from the viewpoint of environmental friendliness. For example, in case of mounting a polyacetal resin valve onto a fuel tank of blow molded multilayer resins of various materials, whose outermost layer is composed of polyethylene resin so as to prevent fuel leakage when the automobile is turned over, it is proposed to fix the valves to the sealing parts through rubber O-rings by screwing. However, a problem of fuel evaporation along the O-rings is not overcome in the proposed procedure, so that the amount of hydrocarbon emission from the automobile cannot be fully controlled.
Integrated structures of polyacetal resin and a different material as integrated together have been so far keenly desired. One of procedures to meet such a desire is to weld a polyacetal resin valve to the polyethylene outermost layer of resin fuel tank, but the polyacetal resin, which is highly crystalline, fails to fulfill sufficient welding at the boundary between the different materials and thus easily peels away under an external force, failing to satisfy the function required for the integrated structures in practice.
To improve the weldability between polyacetal resin and different materials, it has been so far proposed to provide configurations with a mechanical anchor effect such as undercuts, perforations, etc., but provision of proposed configurations complicates shapes and process steps, leading to economical disadvantages and undesirable production efficiency.
Recently, many attempts have been proposed to weld or integrate polyacetal resin with other thermoplastic resins.
For example, JP-A-9-248851 discloses blow molded multilayer articles with an adhesive resin layer of modified olefinic polymer sandwiched between a polyacetal resin layer and another thermoplastic resin layer so as to enhance the bonding strength between both layers, but the bonding strength between the polyacetal resin layer and the modified olefinic polymer layer is practically not satisfactory.
JP-A-2000-8981 discloses a method for integrating a polyethylene resin piece with a polyacetal resin piece using an annular welding part of modified polyolefin resin having polar functional groups, but the welding strength between the annular welding part and the polyacetal resin piece is also considerably low and fails to fulfill practical services.
Furthermore, JP-A-11-320605 discloses a composite molding made by the so called resin insert injection molding, i.e. by providing a polyacetal resin injection molding in a molding, treating the surface skin layer of the molding by flame, etc., and then injection molding another thermoplastic resin thereon, but the bonding strength between the polyacetal resin and the other thermoplastic resin is also still not satisfactory, and furthermore the step of heat treating the surface of polyacetal resin molding with flame, hot air, etc. is complicated and economically efficient.
JP-A-11-320606 discloses a composite molding made by the so called resin insert injection molding, i.e. by providing a polyacetal resin injection molding into a mold and then injection molding polyalkylene terephthalate resin thereon, but conditions for injection molding the polyalkylene terephthalate resin as a secondary member have a low degree of freedom, e.g. unless a plasticizing cylinder temperature, a filling time, etc. are carefully selected, there would be a high possibility to fail to obtain a composite molding with a satisfactory bonding strength, and a complicated problem in setting the molding conditions and a laborious problem in quality control at mass production in molding shops, etc. at the same time.